The present invention relates to a magnetic resonance signal acquiring apparatus and magnetic resonance imaging apparatus, and more particularly to an apparatus for sequentially acquiring magnetic resonance signals for a plurality of views, and an apparatus for producing an image based on the thus-acquired magnetic resonance signals.
In a magnetic resonance imaging (MRI) apparatus, a subject to be imaged is carried into an internal space of a magnet system, i.e., an imaging space in which a static magnetic field is generated, a gradient magnetic field and a high frequency magnetic field are applied to excite spins within the subject to thereby generate magnetic resonance signals, and an image is reconstructed based on the received magnetic resonance signals.
If the installation site of the magnet system is near an electric railroad or the like, passage of an electric train causes magnetic field disturbance. Since the quality of the reconstructed image is degraded if the magnetic resonance signals are affected by the magnetic field disturbance, measures are taken for reducing the effect of the magnetic field disturbance.
One of such measure is to employ a magnetic field generating apparatus for canceling the magnetic field disturbance. This apparatus comprises a magnetic field sensor and a magnetic field generating coil, and is configured to generate a magnetic field by the magnetic field generating coil corresponding to the magnetic field disturbance detected by the magnetic field sensor for canceling the magnetic field disturbance.
The magnetic field generating coil employed is a Helmholtz coil. Three Helmholtz coils each having a coil axis perpendicular to one another are employed to allow magnetic fields in three mutually perpendicular directions to be generated. Six coil loops in the three Helmholtz coils are disposed respectively on six surfaces of a scan room, i.e., on the ceiling, floor, and four lateral walls. Each coil loop has an electric path along the periphery of each surface.
Another measure involves arithmetically calculating the effect of the magnetic field disturbance on the magnetic resonance signals, and correcting the magnetic resonance signals accordingly. Magnetic resonance signals for use in calculating the effect of the magnetic field disturbance are collected separately from those for image reconstruction. The number of repetitions of the signal collection is set so that the disturbance can be detected at a desired temporal resolution.
When the magnetic field generating apparatus for canceling magnetic field disturbance is employed, special construction work is needed to install the apparatus in the scan room. When the effect of magnetic field disturbance is arithmetically calculated from magnetic resonance signals, magnetic resonance signals for use in the calculation must be collected separately from those for image reconstruction a specified number of times, regardless of whether magnetic field disturbance is present. Moreover, the magnitude of magnetic field disturbance that can be canceled or corrected is limited regardless of which technique is employed.